1. Field of the Invention
This invention relates to the shaping of glass sheets and, in particular, to lightweight bending iron shields for forming hot glass sheets to complicated shapes.
2a. Technical Considerations
In the heating and shaping of glass sheets it is common practice to use contoured bending molds that support a flat glass sheet and convey it through a heating lehr. As the temperature of the glass increases and approaches its heat softening temperature, it begins to sag under the force of gravity and conforms to the contours of the mold. If required, the bending molds can include hinged end sections to help facilitate bending portions of the glass sheet to sharp curvatures that could not normally be obtained through normal sag bending techniques. Where windshield glass is being bent, two glass sheets, or doublets, can be conveyed through the lehr on the bending mold and shaped simultaneously.
The trend in automotive styling has been toward the use of glass closures that conform with the shape of the vehicle, requiring more complicated shapes, which in turn require additional bending techniques. These configurations include compound bends such as deep sags or S-shaped curves, wherein the curvature is continuous but changes from one portion of the glass sheet to another.
To effect these configurations, localized heat distribution patterns are established at the bending mold so as to direct more heat to the areas requiring additional sag or small radius bends. In a sag bending operation where the heat source is located above the glass sheet to be shaped, heat absorbers or ballast are used to establish a temperature variation between different portions of the glass sheet to produce the desired curvature. The ballast is placed below the glass sheet at areas that are to remain relatively flat or do not require the small radius bends. The ballast is usually in the form of solid steel plates or heat absorbing metal mesh. The ballast absorbs heat that would otherwise be absorbed by the glass sheet and cause the sheet to sag more than desired. Ballast is generally 1/4" to 1/2" thick or more and can be stacked to increase its effective heat absorbing capacity.
In order to more uniformly heat the top and bottom surfaces of the glass sheet to be shaped, bottom heat can be added below the glass sheet. This is particularly important in instances of simultaneously sag bending glass doublets where the heating must be controlled so that final curved configuration of each glass sheet complements the other. When bottom heat is added, the amount of top heat is generally reduced to balance the total amount of heat required to sag bend the glass sheets to the desired configuration. When lehr heat is supplied from below the glass sheet, shields are used to shade the lower sag areas from the heat source to establish the required temperature differential and retard heat absorption and limit the sag.
It has been found that as the bending molds progress through the heating lehr, thin ballast and/or shields may absorb sufficient heat to begin to reradiate the absorbed heat. As a result the ballast loses its effectiveness as a heat sink and the shields lose their effectiveness as a shade for the glass sheet. When this occurs, heat is directed to areas of low sag due to the ineffective ballast and/or shields, thus disturbing the heat distribution of the bending mold and resulting in glass sheets shaped to the wrong configuration.
It would be advantageous to provide a bending mold wherein the shields can operate to shade selected portions of the glass sheet without loosing their effectiveness as a shield.
2b. Patents of Interest
U.S. Pat. No. 3,020,677 to White teaches the use of water boxes mounted below the glass sheet in a bending mold to generate steam to keep the center of the windshield cool. Heat is provided from both above and below the bending mold as the glass is conveyed through the lehr.
The following list of patents discloses the use of shields and heat absorbers for bending glass sheets:
______________________________________ U.S. Pat. No. Inventor ______________________________________ 3,086,376 Carson et al. 3,086,377 Laflet Jr. 3,137,558 Oberstar 3,186,821 Carson et al. 3,253,898 Laflet Jr. 3,264,082 Golightly 3,265,488 Ross et al. 3,269,822 Carson et al. 3,281,231 McKelvey et al. 3,433,616 Wampler et al. ______________________________________
In each of these patents, wire mesh screens or steel plates are used as heat absorbers to draw heat away from selected portions of an overlying glass sheet. As a result, the glass sheets in the regions of the heat absorbers do not reach the heat softening temperature at the same time the remainder of the glass sheet attains this temperature so that there is less sag in these selected areas. In particular, U.S. Pat. No. 3,186,821 uses plates to act as both shields and heat absorbers to form the glass sheets, i.e., to both shade the glass and to further withdraw heat from the atmosphere adjacent the shaded portions of the sheet to further retard the heating of the shaded portions.